Each embryo combines half the genetic information from its parents – 23 chromosomes from each. One pair of them is responsible for determining sex – girls have two X chromosomes each, and boys have an XY pair. Gender is also determined at conception, based on genes. However, the Y chromosome is not immediately activated. The first five to six weeks take place under the sign of the X chromosome. The development of girls is entirely under the influence of this chromosome alone. When the Y chromosome is also activated in boys, one of its genes has two functions – to suppress certain characteristics of the X chromosome and to provoke the development of the physiological traits that a man should possess. Thus, for example, the ovaries become testicles, the scrotum is formed. The clitoris develops into a penis. Nipples begin to form before the activation of the Y chromosome is the explanation why men also have nipples but not breasts.